366 VEGETABLE PHYSIOLOGY 
for it involves a particular condition of the protoplasm 
with regard to its permeability. This condition may be 
regarded asa kind of effort, the living substance exerting 
some active living influence comparable to the condition 
of almost passive contraction, which is the normal condition 
of various muscular structures in the animal body. The 
effort seems to be directed to reducing the resistance its 
structure offers to the passage of water through it. If it 
is increased, the existing hydrostatic pressure causes an 
excessive escape of water, and the cells become flaccid ; if 
it is relaxed, the normal interchange of water between cells 
is diminished to their detriment, the permeability of the 
protoplasm becoming lessened. 
A further aspect of tone may be seen to depend upon a 
constant and regular supply of oxygen. The function of 
this gas in vegetable life has already been discussed at 
some length in a preceding chapter. We have seen that 
if its access is interfered with the whole organism is for a 
time, if not permanently, upset, all the vital functions 
being thrown into disorder. The power of appreciating 
and responding to stimulation is also lost. 
Another property which vegetable protoplasm possesses, 
and which is of the highest importance in adapting the 
organism to its environment, is what has been termed 
acclimatisation. This is manifested by the fact that after 
long continued applications of a particular stimulus the 
organism ceases to respond to it. This is shown by the 
fact that a plant, accustomed to live in light of but feeble 
intensity, if made to grow in a brighter region, though 
injuriously affected at first, will ultimately thrive in it as 
well as it did before. Similar phenomena in connection 
with temperature have been observed. 
